I created a simple trigger that whenever we create a "mailing" task, after the task is marked "completed" it updates the letter counter on a custom object.

trigger UpdateSentMail_02 on Task (after Insert, after update){ 

double i = 0;

// Tasks that meet criteria, and new tasks to create
Task[] qualifiedTasks = new Task[0], newTasks = new Task[0];
    
// Map of SD Members
   Map<Id,SD_Member__c> members = new Map<Id,SD_Member__c>();

// Find qualifying tasks
for(Task record:Trigger.new)
        if(Record.subject!=null && record.subject.contains('Print Sf Order Submitted') && record.status == 'Completed')
           qualifiedtasks.add(record);
    
    // Obtain member ID values
    for(Task record:qualifiedtasks)
        members.put(record.whatid,null);
   
    // If there are any membes to query, do so.
    if(!members.isempty())
        members.putall([select id, letter_counter__c from SD_Member__c where id in :members.keyset()]);


     for(Task record:qualifiedtasks) {
// Get the current Letter Counter value
   i = members.get(record.whatid).letter_counter__c;
// Increase the Letter Counter by 1
    i=i+1;

        members.get(record.whatid).Send_Monthly_Letter__c=False;
    }
    
     update members.values();

    for(Task record:qualifiedtasks) {
        members.get(record.whatid).Letter_Counter__c=i;
    }
    
    update members.values();
}

 When I run it one record at a time, that is, I go into SF and create each task by hand it runs fine.  But if I try and upload 500 to 1000 tasks using Excel connector or Data loader, nothing happens.

Any ideas?